Microsoft has been providing bootable USB media for Windows 10 if you opt to have a physical media. The image created on the USB stick from the Windows Media Creator is also bootable.
To boot from the USB to start installation from Fusion, the VM needs to have virtual firmware using UEFI. If you have Fusion 10.x, you should be able to choose the firmware as UEFI. If you are on Fusion 8.5.x you need to add the line
firmware="efi"
to the vmx configuration file after creating the custom virtual machine.
Whether Fusion 8.5.x or Fusion 10.x; create a custom virtual machine and (choose UEFI as the virtual firmware for Fusion 10.x).
I am still on Fusion 8.5.x; and last time I had to boot from Windows 10 USB, I had to do this workaround for booting Windows 10 USB in Fusion to work. I had to add a delay in the power up of the firmware. Add the line to the vmx configuration file of the Windows 10 VM
bios.bootDelay = "7000"
That is 7000 milliseconds (or 7 seconds); you can adjust the time. The objective is to have enough time to connect the USB boot device to the Windows 10 VM (from Virtual Machine menu - USB & Bluetooth) after you select you select "Power On to Firmware" from the "Virtual Machine" menu. For the life of me, I couldn't figure out how to make the Windows 10 USB boot device appear as a boot option without this workaround.
On Workstation 12.5.x on Windows I don't need this workaround. Just Power On to Firmware; connect the USB to the VM and enter the EFI setup and then go back to the EFI main menu and the "EFI USB" simply shows up as a boot option in the EFI main menu. Then just select "EFI USB" to boot up and begin Windows 10 installation.
